The present invention relates to cesium dispensers and a process for using them.
Cesium has been used for a long time in the electronics field. In particular, this metal has been used in the past for the production of photosensible surfaces, for example of image intensifiers or photomultiplier tubes.
A novel application field for cesium is in the OLED (Organic Light Emitting Display) screens.
In short, an OLED is formed of a first transparent planar support (of glass or plastics); a second, not necessarily transparent support that may be realized in glass, metal or plastics, essentially planar and parallel to the first support and secured along the perimeter thereof, so as to form a closed space; and a structure in the space that is active in forming an image. The active structure is formed in turn by a first set of linear and reciprocally parallel, transparent electrodes, deposited on the first support; a multilayer of different electroluminescent organic materials comprising at least one layer of electron-conductive material and one layer of an electronic vacancies (also defined in the field as xe2x80x9cholesxe2x80x9d) conductive material deposited on the first set of electrodes; a second set of linear and reciprocally parallel electrodes that are orthogonally oriented with respect to those of the first set and in contact with the opposite side of the multilayer of organic materials, so that the latter is comprised between the two sets of electrodes. For a more detailed description of the structure and operation of OLED screens one can refer for instance to European published patent applications EP-A-0 845 924 and EP-A-0 949 696, Japanese published patent application JP-A-9-078058, and U.S. Pat. No. 6,013,384. Recently it has been ascertained that the doping of one or more layers of the organic multilayer with small amounts of electron-donor metals, in particular cesium, enables reduction of the potential difference to be applied to the sets of electrodes for the functioning of screens, and thus reduction of the energy consumption of the latter.
Due to its high reactivity to atmospheric gases and moisture, cesium is not usually used in industry as pure metal, but rather in the form of its compounds which are stable to air at room temperature.
Some cesium compounds release the metal by simple heating. Among these compounds alloys with silicon or germanium may be cited, as described for example in European published patent application EP-A-0 360 317 and U.S. Pat. No. 5,066,888, as well as the intercalation compound of cesium with graphite, having the formula CsC8, cited in European patent application EP-A-0 130 803. These compounds have, however, no practical application at industrial level.
Cesium dichromate, Cs2Cr2O7, or more commonly cesium chromate, Cs2CrO4, is normally used in industry in mixture with a reducing agent. By heating these mixtures at temperatures generally over 500xc2x0 C., and usually between 550 and 650xc2x0 C., a reaction takes place wherein chromium is reduced to a lower valence, as a consequence of which cesium is released in vapor form. As reducing agents aluminum, silicon or getter alloys, i.e., alloys based on titanium or zirconium with aluminum or one or more transition elements, are generally used. The use of these mixtures is described for example in U.S. Pat. No. 2,117,735.
These compounds are generally introduced into suitable dispensers that are able to retain solid particles of the compounds, but have at least a portion of the surface permeable to cesium vapors. Various forms of dispensers are the object, for instance, of U.S. Pat. Nos. 3,578,834; 3,579,459; 3,598,384; 3,636,302; 3,663,121; and 4,233,936. A further property required for cesium dispensers is not to release gases that are detrimental to the operation of devices where cesium is used during their production.
Cesium chromate and dichromate suffer, however, the disadvantage of containing hexavalent chromium, which may cause irritations by contact, ingestion or respiration and may be carcinogenic in case of protracted exposures.
In production processes of common devices in which cesium is used (image intensifiers or photomultipliers) high temperatures are reached, and only by using chromate and dichromate can one avoid the release of cesium in an early stage of the process. In addition, limited productions are obtained in these cases, and consequently the amounts of chromate that are used are limited too.
The temperatures of production processes of OLEDs are lower in contrast. For these screens productions on a very large scale, on the order of tens of millions of pieces per year, are foreseen. With these production volumes, the safety problems linked with the transportation and use of chromates become significant. In the production of OLEDs it is thus possible and highly desirable not to resort to the use of Cs2CrO4 or Cs2Cr2O7 to evaporate cesium.
An object of the present invention is to provide cesium dispensers particularly suitable for the production of OLED screens, wherein cesium is not present in the form of a chromium salt.
A further object of this invention is to provide a process for the use of cesium dispensers in the production of OLED screens.
These objects are achieved by the present invention, wherein a first aspect relates to cesium dispensers formed by a container able to retain solid particles, but having at least a part of the surface permeable to cesium vapors and containing a mixture of at least one cesium compound and at least one reducing agent, characterized in that the cesium compound is selected among molybdate, tungstate, niobate, tantalate, silicate, and zirconate.